Positron emission tomography (PET) is a specialized radiology procedure that generates and examines three-dimensional images of functional processes in a target organ or tissue of a body. Specifically, in PET studies, a biologically active molecule carrying a radioactive tracer is first introduced to a patient's body. The PET system then detects gamma rays emitted by the tracer and constructs a three-dimensional image of the tracer concentration within the body by analyzing the detected signal. Because the biologically active molecules used in PET studies are natural substrates of metabolism at the target organ or tissue, PET can evaluate the physiology (functionality) and anatomy (structure) of the target organ or tissue, as well as its biochemical properties. Changes in these properties of the target organ or tissue may provide essential information for the identification of the onset of a disease process before any anatomical changes related to the disease become detectable by other diagnostic tests, such as computed tomography (CT) or magnetic resonance imaging (MRI).
Furthermore, the unique high sensitivity of PET—in the picomolar range—allows detection of even minute amounts of radio-labeled markers in vivo, making PET the modality of choice for molecular imaging. In this respect, an important new perspective in the field of nuclear imaging was created by using PET in conjunction with other diagnostic tests to realize simultaneous acquisition of both structural and functional information of the body and provide more definitive information about malignant (cancerous) tumors and other lesions. For example, since the introduction of combined PET/CT (computed tomography) systems about 10 years ago, medical practitioners in the fields of oncology, neurology, cardiology and radiology have been taken advantages of the dual-modality system to construct and analyze three-dimensional functional PET images in comparison with structural x-ray CT images that are obtained almost simultaneously with a same PET/CT scanner in a single session.
To this end, there are many clinical indications where magnetic resonance imaging (MRI) is preferred over CT. For example, MRI offers, compared to CT, better soft tissue contrast and does not use ionizing radiation, thus significantly reducing the overall required radiation doses and associated risk or harm to a patient. Furthermore, in addition to structural imaging, MRI can also be used to visualize functional activity of the body. For example, functional MRI or fMRT, measures changes in blood flow to different parts of the brain. In this type of studies, signals reflecting the blood-oxygen levels in the brain can be reliably used as a proxy for brain activity, because neurons use more oxygen when they are active.
Thus, the current need in the field of non-invasive diagnostic imaging to accurately and transparently combine high resolution, three-dimensional functional PET information with equally high quality morphological and/or functional MRI information within a single device establishes a clear new direction for research and development of next generation multi-modality imaging technology.
A PET/MR hybrid system capable of simultaneous dual-modality imaging would provide many advantages which go far beyond simply combining separately acquired PET and MRI data. These advantages include not only great convenience, flexibility, and improved speed for multi-modality acquisition of more data, but also much simplified logistics of patient management and significantly reduced patient costs. More importantly, simultaneous multi-modality data acquisition and processing ensure far greater accuracy in registration of PET and MRI data, hence providing medical practitioners more detailed and reliable diagnostic information.
However, despite great endeavor in the field, several technical difficulties continue to exist and hinder the realization of full PET/MR integration and real simultaneous data acquisition. Particularly, PET and MRI are two advanced imaging technologies, which require collecting and processing electronic signals that are delicate and prone to interference. For example, a PET detector may contain temperature-sensitive components and thus need a cooling apparatus to maintain a suitable working temperature. Additionally, optimal functionality of a PET detector also relies on precise coupling and communication between its optical and electrical components. Thus, a PET detector may also contain various mechanical parts to hold the components in precise positions relative to one another. However, the cooling apparatus and/or mechanical parts of a PET detector, when placed into a MRI gantry, may significantly disturb homogeneity of the MRI magnetic field. Further, another major challenge exists with the physical constrains on available space when trying to integrate various PET components into a MM system.
Thus, there exists a need in the field to provide an improved PET detector that overcomes the technical challenges mentioned above.